This invention relates to a solid state imaging apparatus utilizing a photodiode array, a MOS transistor array, a charge transfer device such as a CCD (Charge Coupled Device) and a BBD (Bucket Brigade Device) or a combination thereof, and more particularly to such apparatus which stores charges in proportion to the intensity of incident light and produces a sequential video signal.
As is well known, it is preferable to use a solid state area imaging device for a television camera in place of a pick up tube for converting an optical image of an object to an electrical video signal, in the case where a compact size, long life, high stability and a lower power consumption are required. However, there remain problems in utilizing such device as a practical imaging apparatus.
One of the typical problems is the blooming phenomenon. Blooming is especially objectionable in an imaging devices with high sensitivity, since even a usual light source causes overload at high light portions in a low-light-level environment. Blooming is caused by excess charges generated in a photosensor such as a photodiode. To make such a solid state imaging apparatus capable of operating without blooming in the presence of excessive high light portions, several methods of suppressing blooming have been proposed.
One of the practical methods employs an OFD (Over Flow Drain) within the image sensor to absorb excess charges, is shown in Bell Syst. Tech. Journal, Vol. 51, PP. 1923-1926, 1972. But this method has several problems which are as follows:
The first problem is the limitation of the blooming suppression due to the limitation of the OFD's space in the imaging area. The reason thereof is that quantity of excess charge absorption is mainly proportional to the area of the OFD. The second problem is the decrement of the sensitivity of the imaging apparatus, because of the decrement of the effective area of photosensor due to the existence of the OFD.
Another practical method for suppressing the blooming phonomenon is providing such an n-p-n structure in the imaging area so as to absorb excess charges into the n type substrate of the n-p-n structure, such as shown in ISSCC (International Solid-State Circuits Conference) Dig. of Tech. Papers, pp. 192-193, Feb. 1972. This technique also has problems in that the effect of the blooming suppression is not so high because of the spreading of generated excess charges through the n type substrate of the n-p-n structure into adjacent regions.